After reading this article you will learn about:- 1. Definition of Chain Surveying 2. Principle of Chain Surveying 3. Equipment 4. Execution 5. Terms.
Definition of Chain Surveying:
It is the method of surveying in which the area is divided into network of triangles and the sides of the various triangles are measured directly in the field with chain or tape and no angular measurements are taken.
It is the simplest kind of surveying and is most suitable when the area to be surveyed is small in extent and is fairly level and open with simple details. It is unsuitable for large areas and for areas crowded with many details and over difficult country as in such cases the formation of triangles becomes difficult.
Principle of Chain Surveying:
The principle of chain surveying is to divide the area to be surveyed into a network of connected triangles as a triangle is the only simple figure that can be plotted from the lengths of its sides measured in the field. Since triangulation forms the principle of chain surveying, the chain survey is also sometimes called as chain triangulation.
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If the area to be surveyed is triangular in shape and if the lengths and sequence of its three sides are recorded, the plan of the area can be easily drawn.
But if the area has more than three straight boundaries, for example as in fig 3.1 (a) it is no longer sufficient to measures lengths of the sides only. The field measurements must be so arranged that the area can be plotted by laying down triangles. Several arrangements should be made satisfying this condition but only two are given here as shown in fig. 3.1. (b) and (c).
Equipment Required in Chain Surveying:
The equipment required in chain surveying includes the following:
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(i) A chain with a set of ten arrows.
(ii) A metallic tape 20 m or 30 m in length.
(iii) About one dozen ranging rods (the actual number depends upon the extent of survey).
(iv) An offset rod.
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(v) An optical square or a cross-staff.
(vi) A plumb-bob.
(vii) About 2 dozen pegs (the actual number depends upon the extent of survey).
(viii) A mallet or a hammer.
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(ix) A field-book and a good pencil.
(x) Sundries such as chalk, nails, field-glass, etc.
Field-Party:
The field party consists of four persons as given, below:
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(i) Surveyor who is in-charge of the party and records chain ages and offsets etc. in the field-book.
(ii) Two chainmen or tape-men to measure all lines and offsets to different objects such as buildings, roads, hedges, wire fencing, drains etc.
(iii) Flagman to fix and carry the ranging rods and also to fix pegs for stations as directed by the surveyor.
Execution in Chain Surveying:
A chain survey may be executed in the following steps:
(i) Reconnaissance:
The preliminary inspection of the area to be surveyed is called reconnaissance.
The surveyor should walk over the area to be surveyed carefully noting all its details such as buildings, roads, hedges, etc. and also the probable position of the station-points and the chain lines etc. The object is to get an intimate knowledge of the area so as to form an idea regarding the difficulties of work, the time required for the survey work etc.
A rough sketch bearing a general resemblance to the plan of the area and showing the north line should be drawn in the field-book.
A base line should be selected in the heart of the survey and whole of the area should then be divided into triangles. The station-points should be marked in the field-book as well as on the rough sketch.
(ii) Marking Stations:
Having completed the reconnaissance, the survey-stations should be marked on the ground so that they may be readily discovered when required. If the survey is small in extent and can be finished in a single day, a station may be marked by fixing vertically a ranging rod. The rod may be supported by a heap of stones if the ground is hard.
If the survey is extensive, wooden pegs of small size about 3 cm square and 15 cm long are driven into the ground for ordinary soil, while wooden pegs about 5 cm square and 40 cm long are used to denote the stations in soft ground.
In pasture land, a turf should be cut in the form of a triangle of about 50 cm side and a peg fixed in the centre. If the surface is very hard such as a road, street etc., it is necessary to fix nails or spikes driven flush with the surface.
(iii) Locating Stations by Reference Sketches:
After the stations are marked, they should be located by tie measurements essentially from two and preferably from three permanent and well-defined objects in the vicinity of the station. These measurements should be taken precisely and recorded in the field-book by means of a sketch called a reference sketch or location sketch as shown in fig. 3.17.
Reference sketches are necessary to recover the positions of stations in case they are displaced or lost or required at a future date. The referenced stations can be easily restored by swinging arcs with reference points as centres and the respective measurements as radii. The intersection of arcs is the required position of the station-mark.
(iv) Running Survey Lines:
Having finished the preliminary work such as selection, marking and location of stations, chaining may be commenced from the base line and carried throughout all the lines of the frame work as continuously as possible. The process of chaining, taking offsets and booking for each line is repeated separately.
Terms Commonly Used in Chain Surveying:
1. Frame Work:
The system of lines or triangles covering the area to be surveyed is called Frame work or skeleton or survey such as ABCDE in fig. 3.2. The arrangement of triangles depends upon the nature and shape of the area to be surveyed.
Since an equilateral triangle can be more accurately plotted than an obtuse-angled triangle, therefore, as far as possible, the triangles formed in a chain survey should be nearly equilateral. The triangles in which the angles are neither very acute nor very obtuse i.e. all angles are greater than 30° and less than 120° are called well-conditioned or well-shaped triangles and are always preferred in a chain survey.
A triangle which is almost an equilateral one is the best suited for plotting work and is known as the best conditioned triangle. The triangles having angles less than 30° or greater than 120° are known as bad conditioned or ill conditioned triangles and should always be avoided. If however they cannot be avoided, great care must be taken during their chaining and plotting.
2. Survey Stations:
The ends of a chain line denote the survey stations.
These are:
(i) Main survey stations, and
(ii) Subsidiary or tie stations.
(i) Main Survey Stations are the ends of the lines which command the boundary of the survey, such as A, B, C, D and E in fig. 3.2 ; and the lines joining the main stations are known as Main Survey Lines such as AB, BC, CD, DE, EA, AC and AD in fig. 3.2.
(ii) Subsidiary or Tie-Stations are the fixed point selected on the main survey lines when it is necessary to draw the lines to sub-divide the area for locating the interior details such as T1, T2, T3, T4 T5 and d in fig. 3.2.
Selection of Survey Station:
The following points should be kept in view while selecting the stations for the frame work:
(i) A station-point should be located on plain ground so that it is clearly visible from all station-points to which it is connected and gives clear straight lines for measurement.
(ii) The main triangles should be so large as is consistent with the features of the ground. These should be sub-divided by the lines if necessary to bring the objects within easy reach of chain lines so that the work is done according to the principle “work from whole to the part.”
(iii) The sides of the larger triangles should pass as close as possible and as parallel as possible to the important buildings, roads, etc., so as to avoid long offsets and to reduce the number of tie lines.
(iv) The triangles should as far as possible be best conditioned and ill conditioned triangles should be strictly avoided.
(v) Each triangle should be provided with at least one check line.
(vi) Station points should not be on thoroughfare.
3. Base Line:
A line which is generally longest of all the survey lines and upon which the entire frame work is built up is known as a base line such as AD in fig. 3.2. It generally runs in the centre of the area to be surveyed and should be laid off on the level ground.
It is very important line and since the entire accuracy of the survey work depends upon its accuracy and straightness, therefore, it should be measured accurately twice or thrice by independent methods and its straightness should also be ensured. In large surveys or where convenient, two base lines should be run in the form of a cross (x) through the centre of the area.
4. Check Line:
A line which is used to check or prove the accuracy of the frame work as well as that of the plotting work is known as a check line or a proof line such as BT3, CT2, and Dd in fig. 3.2. It is a line which runs from apex of a triangle to any other fixed points on any two sides of a triangle. If while plotting, the length of this line on the plan agrees to the length measured in the field, then the work is correct and thus the accuracy of the triangle is checked or proved.
5. Tie Line:
A line joining two tie stations is known as a tie line such as T1, T2, T2, T3, T4, T5 in fig. 3.2. It is run to take the interior details which are far away from the main lines and also to avoid long offsets. It can also serve the purpose of a check line.